Method of preventing the development of stretcher strains in metal sheets



Patented Nov. 15, 1927. v

UNITED STATES PATENT OFFICE.

GEORGE L. KELLEY, .OF PHILADELPH IA, PENNSYLVANIA, ASSIGNOR ,TO. EDWARD G.

BUDD MANUFACTURING COMPANY, IOBATION OF PENNSYLVANIA.

or PHILADELPHIA, rnmvsvnvama, A con METHOD OF PREVENTING THE DEVELOPIIgIINS'T OF STRETGHER STRAINS IN METAL SH E This invention relates to a method of preventing the development of stretcher strains in metal sheets.

The object of this invention is to provide a process of working metal sheets to prevent the formation or development thereof of irregularities usually termed stretcher strains, stretching, strains or wormy metal.

A further object is to provide a. process and mode of operatiomof the nature and for the purpose referred to, which is s mple, efficient, and capable of being carried out in many different ways for practical utilization.

Other objects of the invention will appear more fully hereinafter.

The invention consists substantially in the method of operation hereinafter more fully set forth, as illustrated in the accompanying drawings, and finally pointed out in the appended claims.

In the accompanying drawings, wherein I have shown various arrangements of apparatus suitable for carrying out the objects and purposes of my invention Figure 1 is a view in vertical, central,

longitudinal section, somewhat diagrammatic, of one form of apparatus suitable for carrying out my invention, and showing the metal sheet in position ready to be worked in accordance with my invention.

Fig. 2 is a similar view of the apparatus shown in Fig. 1, showing the metal sheet after the inauguration of the working thereof in accordance with my invention.

Figs. 3, 4 and 5 are similar views of other forms of apparatus suitable for use in carrying out my invention.

Fig. 6 is a diagrammatic vertical cross section of a die stamping operation and the stamping produced thereb Practical experience in the use of metal sheets in the manufacture of various articles has shown that slight irregularities frequently occur or develop in the surface of the sheet, particularly after the sheet has been subjected to an operation which tends to stretch the metal, as, for instance, when the sheet is subjected to the action of pressin the surface.

Application filed January 24, 1924. Serial No 688,249.

ing or forming dies in which a slight stretching or drawing action is imposed on the metal in shaping it to the desired irregular or other cont-our. These surface irregularities are known as stretcher strains, stretching strains or wormymetal, and are exceedingly objectionable, particularly where the surface of the sheet in which they occur is to receive a. finishor polish, such, for example, as stampings employed in the manufacture of automobile bodies. Where such surface blemishes or irregularities occur in a metal sheet or stamping, it is tedious and very expensive work to remove them, requiring filing, grinding, hammering or polishing operations, which consume time and labor, and which very greatly add to the cost of production of the stamped or pressed article.

Various expedients have heretofore been resorted to in the endeavor to avoid the formation or appearance of such irregularities in metal sheet surfaces, and particularly have so-called roller leveler machines been proposed for this purpose, wherein the metal sheets are passed between rolls which have been arranged in various ways in relation toeach other,.with the idea and purpose of imparting bends alternately in opposite directions in the metal sheet preparatory to working the sheet into the desired article to be manufactured. Such er:- pedients, and others which have been proposed, have not proven to be satisfactory, or have failed to attain the desired results.

i have discovered that by first working the metal sheet progressively throughout its length and area by mechanical manipulation so as to flex or bend it far beyond the limits of its elasticity, and then subjecting it to the roller leveler machine, or otherwise straightening it, and using the worked and st raightcned sheet in the forming or pressing dies before the metal of the worked and straightened sheet regains its initial condition, I am enabled to eliminate the formation or appearance of the irregularities the surface of the sheet when it is subjected to the action of the forming or pressing dies.

In carrying out my invention, the metal sheet may be worked, that is, flexed or bent far beyond its limit of elasticity, by mechanical manipulation applied manually, or in various arrangements of apparatus; and the subsequent working of the sheetto straighten it may likewise be accomplished manually or by mechanical operations, the important and essential feature being that the pi" liminary r initial working of the metal of the sheet be accomplished so as to cause thc,shcet to be flexed or bent to a degree or extt it beyond that attained by the roller lc'vclers icrc-tofore employed, and that such working be accomplished progressively throughout the entire area of the sheet, be-

fore, and in addition to, the levelcr operations.

According to one mode of carrying out my invention, the metal sheet is formed into a sharp or acute bend at or near one end thereof, the bend extending transversely across the entire sheet and in symmetrical relation to the adjacent edge of the sheet, and then the line of such bond is progressively advanced throughout the" length of the sheet. This bending action is what I term the initial or additional working of the sheet, the bending or fiexin of the sheet bein carried to a considerab e degree he yon the limit of elasticity. This results in lacing the metal of the sheet in such condition that when it is subsequently passed through a roller leveler, or is otherwise worked to straighten it, and is finally subjected to the action of forming or pressing dies, there is no tendency of the wormy metal, stretcher strains, or the like blemishes or irregularities, to appear or develop in the surface of the presse or formed article. After the metal sheet has been initially worked in the manner referred to, it is important and necessary to straighten out the sheet so that it is in convenient form to be applied in a forming or pressing dle. This straightening operation, as in the case of the initial working operation, may likewise be accomplished manually or by the use of suitable tools, or it may beaccomplished by suitable apparatus such as the ordinary roller leveler. It is also important that the Worked and straightened sheet be subjected to the action of the forming or pressing die before the metal of the sheet resumes its initial condition. That is to say, after the metal sheet has been initially worked and then subjected to a roller leveler or otherwise straightened, there exists an inherent tendency of the metal thereof to return or restore itself to its initial condition. Consequently, in order to secure the maximum advantage to be derived from the initial working of the sheet in accordance with my invention, and its subsequent working to straighten it, there should not be a sufiicient lapse of time after such working and subsequent straightening of the sheet before it is used in the formin or pressing die. I have found it referable to subject the initially worked and subsequent- 1y straightened sheet to the forming or pressing die operation within an hour after the sheet has been initially worked and then straightened, this elapsed time, however, is dependent more or less upon the thickness of the sheet, and the degree or extent to which the bending or other form of initial working is carried. I do not desire, therefore, to limit or confine myself in this respect. It will be understood, of course, that the straightening operation by roller levelers or otherwise after the initial flexing or bending of the sheet adds to the effects of the initial working operation and contributes to the work of placin the sheet in such condition as to prevent the appearance or production of the surface irregularities referred to, as well as to permit it to be conveniently received in the forming dies.

The initial mechanical n'ianipulations of the sheet referred to, whether accomplished manually or by suitable apparatus, including the bending and pro ressive advancement of the bend throug iout the area of the sheet, and the subsequent roller lever or straightening operation, permits a heavier or more secure initial working action on the sheet than would otherwise be possible, that is, permits the bending to be carried further beyond-the limit of elasticity of the metal than has been accomplished heretofore by leveling rollers, and this results in eater freedom of the initially worked an subsequently straightened sheet from the surface irregularities, wormy metal, stretcher strains and the like which are to be eliminated.

The followin brief theoretical discussion offers some explanation of the operation of the invention and its effects upon the metal. When metal is cold worked in mass far beyond the limit of its elasticity for a period of some hours duration after being so worked it shows no yield point in its characteristic curve, this curve being of regular outline all the way u to the point of ultimate strength. Coinci ent with the changes in elasticity characteristics brought about by this cold work or overstrain is a change in the elastic properties of the metal until a point approaching a disappearance of the yield point is reached where the metal partakes 111 general of that characteristic known as cold plasticity. Always when material is relatively in this general state of cold plasticity, the occurrence of stretcher strains 1s precluded altogether. By introducing initially this characteristic of cold plasticity, it becomes possible to work the metal to a sufficiently high degree and yet give a roller leveler its full chance to straighten out the corrugations resulting from the cold working at the same time it straightens out the ordinary surface irregularities for which the usual leveling operation is intended. The greater the degree to which the working for cold plastlcity is carried out, the longer the period of reversion during which the metal may be stamped without the formation of stretching strains or worms.

In Figs. 1 and 2 I have shown one form of apparatus suitable for effecting the mechanical initial working and subsequent straightening of the sheet. This consists of a frame in which are mounted a series of upper rolls and a series of lower rolls, between which sets of rolls the metal sheet 10 to be worked is designed to pass. -The'initial pair of rolls 11, anupper and a lower, are disposed the one immediately above the other with their axes in vertically aligned parallel relation. Likewise the last pair of rolls.12, are similarly disposed with relation to each other. These twolpairs of rolls are what may be called pinch rolls. The intermediate upper rolls 13, are disposed in staggered relation with respect to the intermedlate lower rolls 14. This arrangement constitutes one form of ordinary roller leveler apparatus. In advance of the initial pinch rolls 11, is arranged an auxiliary roll 15, which is movable so as to be broughtinto contact with the surface of the sheet 10 after the front end of the sheet has been engaged between the pinch rolls 11, to bend the sheet sharply or acutely throughout its transverse width on a line adjacent and parallel to the-front edge of the sheet, as shown in Fig. 2. Now, while the sheet is maintained in this bent position, it is advanced through the rolls 11', thereby effecting the desired initial working of the sheet progressively throughout its entire area and length. The action of the staggered rolls, 13, 14, and the pinch rolls, 12, on the sheet, results not only in additionally workway, such, for example, as by means of a piston and piston rod operating in a pressure actuated cylinder, 16, and connected, through suitable connections, to, or operating on, the bending roll to shift it into and out of its sheet-bending position. It will be understood. of course, that all or any of the rolls may be power driven. Ordinarily it is sufiicient to drive the pinch rolls and the lower, or the upper, intermediate rolls. But my present invention is not concerned with such mechanical details.

In Fig. 3 I have indicated diagrammatically another arrangement suitable for carrying out my invention, wherein a pair of rolls, 17, are disposed on opposite sides of the sheet 10 and are soconnected together and mounted that by swinging the arms, 18, connecting them, said rolls are simultaneously moved into and out of bending relation and contact with the op osite surfaces of the sheet, thereby accomplishing the initial working of the sheet throughout its area and length as the sheet is advanced between and past said rolls. In thisinstance the rolls 17 may be shifted in any suitable manner, as for exam le by the piston rod connection and cylin er, 16. T e pinch rolls, 11' and 20, receive and deliver the sheet, while the mtermediate rolls, 21, receive the bent and initially worked sheet and act thereon to 'ad- -I ditionally work it as well as to straighten it.

All of the rolls, or any of them, as well as the movable rolls, 17, may be power driven.

In Fig. 4 is shown another arrangement in which the process of my invention may be carried out, wherein the auxiliary sheet bending rolls, 22, to accomplish the progressive initial working of the sheet throughout its area and length, are disposed on opposite sides of the sheet out of line with each other and are independently moved into or out of bending engagement with the sheet b the power cylinders, 23, 23, and piston ro connections. The pinch rolls, 24, 25, serve to receive and deliver the sheet while the intermediate rolls, 26, impart additional working to the sheet as well as straightening it.

Fig. 5 indicates still another form of apparatus suitable for carrying out the process of my invention. In this case two pairs of pinch rolls, 27, 28,'and intermediate stag gered rolls, 29, are employed and an aux lliary or initial working or bending roll, 30, is carried by swinging arms, 31, so as to be moved into and out of bending relation with respect to the sheet.

After the foregoing steps of my invention have been completed, and while the general state of cold plasticity still persists, the material is die drawn or die stamped by any of the many Well known processes or machines, such for example, as that illustrated in the vertical section of Fig. 6 which is so well known as to be self-explanatory.

Many other specific forms of apparatus may be equally well adapted for use in carrying out the process of my invention.

I have found in the practical use of the process above described that very important and valuable economies are achieved in that the objectionable surface irregularities in metal sheets are eliminated, thereby saving the cost and expense of removing them and permitting the production of sheet metal stampings having surfaces free from such irregularities.

Having now set forth the ob ects and nature of my invention, and various ways in which the same may be carried into practice, what I claim as new and useful and of my own invention, and desire to secure by Letters Patent, is:

1. The process for preventlng, in die drawing of sheet steel and the hire, the development of stretcher strains and other 1rregularities, which consists in subjecting the material shortly before drawing to a cold distortion greater than that which can be effected by a standard roller leveler and independently of the standard leveling operation, whereby the sheet is reduced to a state of cold plasticity, thereafter straightening the distorted material by a leveling operation, and die drawing the same while the state of cold plasticity persists.

2. The process of preventing, in die drawing sheet steel and the like, the development of stretcher strains and other such irregularities, which consists in sub ect1ng the maial, shortly before drawing, to a progressive bending through a relatively great angle as compared with the bending angles of a standard staggered roll leveling o'peration, whereby the sheet is reduced to a general state of cold plasticity throughout, and thereafter additionally subjecting the material to a pro ressive bending through the relatively sma ler angles of a staggered roll leveling operation, whereby the sheet is straightened, and thereupon die drawing the same.

3. The method of preventing, in die drawing of sheet steel and the like, the develop ment of stretcher strains and other such irregularities, which consists in subjecting the material shortly before drawing to a standard staggered roll leveling operation to straighten the same, but adding to the cold working of the leveling operation, and independently thereof, additional cold working sufiicient to place the material relatively in a general state of cold plasticity, and die drawing the material while said state persists.

In testimony whereof, I affix my signature.

GEORGE L. KELLEY. 

